The present invention relates to a liquid crystal display device employing a vertically alignment mode.
Conventionally, liquid crystal devices have been widely used as a screen for word processors and computers, and recently have been pervasive as a screen for televisions. Most of the liquid crystal display devices employ a TN (Twisted Nematic) mode. However, the TN mode liquid crystal display device has problems that contrast decreases and tone characteristic inversion is likely to occur when viewed on an angle.
To improve a viewing angle characteristic in an angled direction, a liquid crystal display device employing a VA (Vertically Alignment) mode has come to receive attention in these years. For example, a VA mode liquid crystal display device is disclosed in U.S. Pat. No. 6,384,889 (patented on May 7, 2002; hereinafter referred to as conventional example) corresponding to Japanese Laid-Open Patent Application No. 2000-47251 (Tokukai 2000-47251; published on Feb. 18, 2000). A liquid crystal cell of the VA mode liquid crystal device is composed of vertical alignment layers and a nematic liquid crystal having a negative dielectric anisotropy.
In the VA mode liquid crystal display device, liquid crystal molecules are aligned in a vertical direction under no applied voltage. When linearly polarized light from a polarization plate enters the liquid crystal layer having the liquid crystal molecules in this state, the light leaves the liquid crystal layer as linearly polarized light without changing the state of polarization, because the liquid crystal layer has almost no birefringence anisotropy. The linearly polarized light is then absorbed by a polarization plate located on the other side of the liquid crystal layer. As a result, the liquid crystal display device can produce black display.
On the other hand, when a voltage is applied, the liquid crystal molecules in the liquid crystal layer are tilted according to the applied voltage. Here, when the liquid crystal molecules are radially aligned as shown in the conventional example for example, the aligned direction of the liquid crystal molecules continuously varies even within a picture element region.
Further, among these types of liquid crystal display devices, there has been a liquid crystal display device in which a chiral dopant is added to vary the alignment of the liquid crystal molecules in a spiral manner along the thickness direction of the liquid crystal layer, as in the normal twisted alignment. This reduces a dark field portion, thereby improving brightness of the liquid crystal device.
In the foregoing conventional example, (U.S. Pat. No. 6,384,889), as described in the first paragraph of EXAMPLE 7 (corresponding to paragraph [0039] in Tokukai 2000-47251) for example, a chiral dopant is added so that a chiral pitch of 18 [xcexcm], which is about four times the cell thickness, is obtained, and a twist angle is set to be about 90 degrees under applied voltage. Under these conditions, a serious quenching pattern remains over a large area and with high intensity, resulting in decrease in transmission intensity and decrease in brightness.
Further, the liquid crystal display device disclosed in U.S. patent application Publication No. 0,036,740 (published on Mar. 28, 2002) includes a structure in which liquid crystal has a twisted structure for the stable alignment of the liquid crystal. However, this publication does not have a notion of eliminating the quenching pattern, or improving the transmittance by eliminating the quenching pattern. Further, the publication does not disclose optimizing the quenching pattern or transmittance.
An object of the present invention is to provide a liquid crystal display device of a vertical alignment mode having a high transmission intensity, i.e., high brightness, the invention realizing the liquid crystal display device (A) by reducing a quenching pattern, which is generated according to a relationship between (i) directions of polarization axes of two polarization plates which are arranged in a cross nicole manner and (ii) an alignment direction of liquid crystal molecules under applied voltage, to such a degree as to be completely unrecognizable for a user, and (B) by determining optical physical property values that can maximize the transmission intensity.
In order to achieve the foregoing object, the present invention provides a liquid crystal display device which includes a first substrate, a second substrate, and a liquid crystal layer between the first substrate and the second substrate, the liquid crystal layer being vertically aligned when no voltage is applied across (i) a first electrode provided on the first substrate and (ii) a second electrode provided on the second substrate so as to face the first electrode via the liquid crystal layer, the liquid crystal layer having a twisted structure and being aligned parallel to the substrates when a voltage is applied across the first electrode and the second electrode, whereby the liquid crystal display device has a defined value for d/p between 0.0021xc3x97(Vmax)2xe2x88x920.0458xc3x97(Vmax)+0.65 and 0.0021xc3x97(Vmax)2xe2x88x920.0458xc3x97(Vmax)+0.50, and has a defined value for dxc2x7xcex94n/xcex between xe2x88x920.00026xc3x97(Vmax)3+0.016xc3x97(Vmax)2xe2x88x920.2281xc3x97(Vmax)+2.124 and xe2x88x920.00026xc3x97(Vmax)3+0.016xc3x97(Vmax)2xe2x88x920.2281xc3x97(Vmax)+1.7603, where d/p is the ratio of a thickness d of the liquid crystal layer with respect to the natural twist pitch p of a liquid crystal, Vmax [V] is the maximum applied effective voltage across the first electrode and the second electrode, and xcex94n is the refractive anisotropy of the liquid crystal layer.
With this arrangement, when no voltage is applied across the first and the second electrodes, the liquid crystal molecules in the liquid crystal layer are vertically aligned. This causes no birefringent effect or optical rotatory effect, so that the light passes through the liquid crystal layer and leaves the liquid crystal layer almost unaffected. On the other hand, when a voltage is applied, the liquid crystal layer assumes a twisted structure and the liquid crystal molecules are aligned parallel to the substrates, thereby causing the birefringent effect and optical rotatory effect. As a result, the state of light that leaves the liquid crystal layer can be changed depending on whether or not a voltage is applied, enabling the display state to be changed in accordance with the voltage.
The inventors of the present invention have diligently worked on a liquid crystal display device of a vertical alignment mode (A) to reduce a quenching pattern, which is generated according to a relationship between (i) directions of polarization axes of polarization plates which are arranged in a cross nicole manner and (ii) an alignment direction of liquid crystal molecules when a voltage is applied, to such a degree as to be unrecognizable for a user, and (B) to determine optical physical property values that can maximize the transmission intensity. As a result, the inventors of the present invention have accomplished the present invention by finding that (1) the liquid crystal molecules in the vicinity of the substrates remain vertically aligned even under applied voltage by the pre-applied regulating forces acting on the liquid crystal molecules, because the liquid crystal molecules are vertically aligned under no applied voltage in the liquid crystal display device of a vertical alignment mode, (2) the thickness of a portion of the liquid crystal molecules where the birefringent effect and optical rotatory effect are generated is accordingly thinner than the actual thickness of the liquid crystal layer, and (3) the thickness of this portion of the liquid crystal molecules varies in accordance with an applied voltage.
Namely, in the liquid crystal display device of the present invention, the liquid crystal layer has defined values for d/p and dxc2x7xcex94n/xcex that respectively fall in the foregoing ranges, i.e., the values according to the maximum applied effective voltage on the first and second electrodes, and that have been set by taking into account the inclined alignment of the liquid crystal molecules in the vicinity of the substrates under applied voltage. As a result, it is possible to suppress the quenching pattern to a such degree as to be unrecognizable for a user, thereby realizing a liquid crystal display device capable of realizing brighter display with higher display quality, compared with a device in which the quenching pattern is recognized. Here, the quenching pattern that can be reduced in the present invention is a quenching pattern that generates on the electrodes within picture elements.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.